The present invention relates to modifications of the DNA sequencing method developed by F. Sanger (Sanger, F., Nicklen, S., Coulson, A. R. (1977) Proc. Natl. Acac. Sci. U.S.A. 74, 5463-5467) as well as to novel enzymes which can be used for DNA sequencing. The Sanger sequencing method is based on in vitro DNA synthesis reactions in the presence of a primed DNA template, 2'-deoxyribonucleoside triphosphates (dNTPs, see FIG. 1), and 2',3'-dideoxyribonucleoside triphosphates (ddNTPs, FIG. 1). The latter, when incorporated by a DNA polymerase into a polynucleotide chain, terminate further chain elongation. The DNA products are thus a series of polynucleotide chains complementary to the template and terminated with specific dideoxynucleotides. The DNA sequencing products can be separated by size and the pattern of the products gives the DNA sequence.
In principle, DNA polymerases from a variety of organisms and a variety of chain-terminating nucleotides should be useful to sequence DNA. In practice, few DNA polymerases and chain-terminating nucleotides have been found to be suitable for this purpose. As an example of a DNA sequencing polymerase, the development of bacteriophage T7 DNA polymerase, Sequenace.TM., will be reviewed (Tabor, S., and Richardson, C. C. (1990) J. Biol. Chem. 265, 8322-8328). In order to obtain an unambiguous DNA sequence it is necessary that the majority of sequencing products terminate with a dideoxynucleotide and that all the sequencing products are represented equally. Two phage T7 DNA polymerase activities degrade DNA sequencing products and, thus, these activities must be eliminated in order to prevent degradation of dideoxynucleotide-terminated sequencing products. One activity, 3'.fwdarw.5' exonuclease activity, was removed by constructing an exonuclease deficient variant of T7 DNA polymerase. T7 DNA polymerase also has pyrophosphorolytic activity which can degrade the sequencing products. Pyrophosphatase was added to degrade pyrophosphate produced in the DNA sequencing reactions; without pyrophosphate, there is no pyrophosphorolysis. A further refinement of the sequencing reactions was to use Mn.sup.2+ in place of Mg.sup.2+ which resulted in a more equal distribution of reaction products. Although this brief review of the development of T7 DNA polymerase into a sequencing polymerase is a simplification, the review illustrates the point that modification of a natural DNA polymerase as well as development of reaction conditions is required in order to obtain high quality DNA sequence information using the chain-terminating sequencing method.
Optimal DNA sequencing conditions using the chain-terminating method have not yet been achieved. Ambiguous sequencing information is still observed which necessitates determining the DNA sequence of both DNA strands. Also, the use of Mn.sup.2+ in place of Mg.sup.2+ increases the amount of DNA template required for sequencing reactions. Thus it would be advantageous to develop novel methods that would improve or complement existing sequencing procedures.
The wild type T4 DNA polymerase gene has been cloned and the protein product expressed (Lin, T.-C., Rush, J. R., Spicer, E. K., and Konigsberg, W. H. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 7000-7004; U.S. Patent 4,935,361 to Lin et al.) and E. coli DNA polymerase II has been cloned and expressed (Bonner, C. A., Hays, S., McEntee, K., and Goodman, M. F. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 7663-7667). Standard oligonucleotide-directed mutagenesis techniques have been used to construct novel forms of T4 DNA polymerase and E. coli DNA polymerase II. Thus, the means exist to economically prepare large quantities of wild type and variant T4 DNA polymerase and E. coli DNA polymerase II.
Another aspect of the invention is to use genetic analysis to identify DNA polymerases with properties useful for DNA sequencing. T4 DNA polymerase is one of the most extensively genetically characterized DNA polymerases (Reha-Krantz, L. J. (1993) In Molecular Biology of Bacteriophage T4, ed. Karam J., American Association for Microbiology, in press); hence, some mutant DNA polymerases already identified may have properties useful for DNA sequencing and new mutants can be isolated directly. A method to isolate novel T4 DNA polymerases with useful DNA sequencing properties would be of additional utility.